1. Field of the Invention
This invention relates to an image forming apparatus of the electrophotographic type called, for example, a full color copying apparatus or a full color printer, and particularly to an image forming apparatus provided with a plurality of image bearing members for forming a multiple image by transferring color toner images to a transfer material sequentially.
2. Related Background Art
FIG. 9 of the accompanying drawings shows a full color image forming apparatus which is a full color copying apparatus of so-called four-drum construction using four photosensitive drums.
Referring to FIG. 9, the full color image forming apparatus 30 is such that an original is placed on an automatic original conveying device DF and when a start button (not shown) is depressed by a user, a sheet (transfer material) S is fed from a cassette 1a or a cassette 1b. The sheet S stands by at regist rollers 2 in order to take timing with an image forming portion. In the meantime, the original is conveyed onto an original supporting table 3 and is scanned by an optical system 4 and is read into a CCD. Here, the image of the original is resolved into components, i.e., a yellow image (Y), a magenta image (M), a cyan image (C) and a black image (Bk).
A laser beam is then turned on and off for each component, whereby in image forming portions for Y, M, C and Bk, development is effected on photosensitive drums (image bearing members) 5Y, 5M, 5C and 5Bk.
In the image forming portions, there are primary chargers 6Y, 6M, 6C and 6Bk for uniformly charging the photosensitive drums 5Y, 5M, 5C and 5Bk, respectively, developing devices 9Y, 9M, 9C and 9Bk for developing latent images into visible images, transfer chargers 7Y, 7M, 7C and 7Bk for transferring the visible images onto the sheet S, and cleaning devices 8Y, 8M, 8C and 8Bk for removing any residual toners on the photosensitive drums 5Y, 5M, 5C and 5Bk.
Also, there is disposed a transfer belt 19 extending through the image forming portions for Y, M, C and Bk, and each color image is transferred to the sheet S while the sheet S standing by at the regist rollers 2 is electrostatically attracted onto and conveyed on the transfer belt 19 in timed relationship with the development on the photosensitive drums. Thus, the image formation in a superposed state is sequentially effected on the sheet S. Thereafter, the sheet S is conveyed to a fixating device 10, where the toner images are melted and fixated, and the sheet S is discharged out of the apparatus onto a tray 20.
When both-surface images are to be formed, after the first surface of the sheet S has been subjected to the fixating step by the fixating device 10, the sheet S is directed downwardly by a flapper 12 located at the entrance to a both-surface conveying path, is reversed by a reversing portion 15, passes along a conveying path 32 and is placed onto an intermediate tray 31. Thereafter, the original is interchanged, sheet resupply is effected and the sheet S is again sent to the image forming portions, where images are formed on the second surface of the sheet S, and the sheet S passes through the fixating device 10 and is placed onto the tray 20 outside the apparatus.
In the apparatus shown in FIG. 9, a drive system as shown in FIG. 10 of the accompanying drawings has heretofore been adopted as a driving construction for the plurality of photosensitive drums 5. A drum driving pulley 41 is mounted on the central shaft of each photosensitive drum 5, and those pulleys are driven by a timing belt 42 for driving by rotating a drive pulley 43a mounted on the output shaft of a relatively inexpensive DC motor 43 from which a rotation output of good accuracy is obtained during high-speed rotation Usually the highly accurate high speed rotation output of the DC motor 43 is decelerated by the pulley/belt and is transmitted to each photosensitive drum 5.
FIG. 11 shows the drum driving portion shown in FIG. 10 in greater detail. The photosensitive drum 5 is coaxially supported on a drum shaft 38 journalled by bearings 39b and 39a attached to a body side plate 34 and an aligning plate 36, respectively, mounted on the other body side plate 35, through drum flanges 5a and 5b, and is integrally fixed to the drum shaft 38 by a stopper 37. That is, the construction in which the plurality of photosensitive drums which are a plurality of driven members are driven by a single drive source (DC motor) rotated at high speed, through mechanical elements such as a timing belt and pulleys or gears or the like (not shown) is adopted in almost all of the full color copying apparatuses or color printers according to the prior art.
Also, the DC motor is usually provided with means such as an encoder for making the number of output revolutions of the motor high and detecting the rotated state in the interior of the motor so that predetermined rotation accuracy may be obtained, and the control of the rotation thereof is effected on the basis of a detected signal.
However, the drum driving construction according to the above-described example of the prior art has suffered from disadvantages arising from what will be described hereinafter.
When the photosensitive drums 5, the drum flanges 5a, 5b and the drum shafts 38 are to be assembled, the respective members are sometimes in contact with one another only at a certain point and assembled in an inclined state due to the backlash at the fitted portions of the respective members or the inconvenience of the shapes thereof (because it is difficult to make them into a completely circular shape). This is such as shown in FIG. 12 of the accompanying drawings. That is, as shown in FIG. 12, in the above-described state, there is created a deviation between the center 38.sub.0 of the drum shaft 38 and the center 5.sub.0 of the photosensitive drum 5, in other words, between the center of the drive transmitting member and the center of the driven member. As a result, the drum shaft 38 and the drum flange 5a (5b) contact with each other at a point C.sub.1, and the drum flange 5a (5b) and the photosensitive drum 5 contact with each other at a point C.sub.2.
The graph of FIG. 13 of the accompanying drawings shows a change in the distance from the center 38.sub.0 of the drum shaft 38 to the surface of the photosensitive drum 5 in the direction of arrow .theta. in FIG. 12. When with the drum shaft 38 as the reference, the photsensitive drum 5 is rotated in the state of the distance change shown in the graph of FIG. 13, there is a member (a cleaning blade or the like) directly contacting with the photosensitive drum 5 around the photosensitive drum 5. Therefore, the drum shaft 38 receives a load from that member through the photosensitive drum 5, and the rotational speed of the drum shaft 38 causes a periodic fluctuation relative to a desired speed during each rotation of the photosensitive drum 5.
When at that time, the drum driving of the apparatus is that shown in FIG. 10, a mechanical element (the timing belt 42 for driving or the like) is interposed between the drive source (the DC motor 43) and the driven member (the drum driving pulley 41), whereby it is difficult for the speed fluctuation of each photosensitive drum 5 to be quickly transmitted to speed detecting means in the drive source without delay. Therefore, each photosensitive drum 5 has been rotated while causing a speed fluctuation during each one full rotation thereof to thereby effect image formation. An image obtained under such a situation will now be described.
In the apparatus effecting the drum driving shown in FIG. 10, grating images at equal intervals for two surfaces are formed by the use of two photosensitive drums 5, as shown in FIG. 14 of the accompanying drawings, and multiplex transfer is effected with the images deviated by a certain distance (half pitch) on a sheet of paper, and the graph of FIG. 15 of the accompanying drawings shows the distance between the first and second gratings from the leading end to the trailing end of the sheet. In FIG. 14, there are shown the state of the image on the sheet at that time and measured portions (a.sub.1, a.sub.2, . . . , b.sub.1, b.sub.2, . . . ). In FIG. 15, it is seen that the distance between adjacent gratings changes with a certain constant period. The change in the behavior of a line (envelope) linking the upper points in this graph together (the changes in a.sub.1, a.sub.2, . . . in FIG. 14) is the amount of deviation between two images, and that amount is great and changes periodically.
FIGS. 16A and 16B of the accompanying drawings show images similar to FIG. 15 with one of the fixed positions thereof relative to the drum shafts of two photosensitive drums (in FIG. 12, the relation to the position of the center 5.sub.0 of the photosensitive drum 5 and the position of the center 38.sub.0 of the drum shaft 38) being a certain position and the other being an image obtained when a part thereof has been changed, and in FIG. 16B, the fixed position of one of the photosensitive drums is replaced with that of FIG. 16A. From FIGS. 16A and 16B, it is seen that the behavior of the images fluctuates from location to location and the amount thereof also changes.
That is, when image formation is to be effected in the apparatus adopting the drive system shown in FIG. 10, a stable image (a pattern in which the peaks/valleys shown in FIGS. 16A and 16B appear constantly) is obtained only when design is made such that the relation of each photosensitive drum to the drum shaft always becomes constant. That is, a constant method of assembly is always required during the assembly of the apparatus and the interchange of the photosensitive drums.
The periodicity of the image shown in FIG. 15 will now be described with reference to FIGS. 17, 18A and 18B of the accompanying drawings.
FIG. 17 shows a measuring method carried out to find out the periodicity. The laser beam of a laser Doppler speedometer is applied near to the same points on the side surface (the measuring points indicated by X in FIG. 17) on drum driving pulleys 41 made integral with two photosensitive drums 5 on which image formation has been effected, to thereby detect speed waveforms (V.sub.3 and V.sub.4 in FIG. 17) corresponding to a constant time at the measuring points at the same times. FIG. 18A shows the mutual correlation between V.sub.3 and V.sub.4 detected by a frequency analyzer, and FIG. 18B shows what has been obtained by frequency-analyzing it.
That is, in the graph shown in FIG. 18B, the frequency component of a portion in which there is a peak affects the speed waveforms of the two photosensitive drums, and that frequency is a frequency component created by one full rotation of the photosensitive drum. That is, the speed fluctuation created by each one full rotation of the photosensitive drum relates between the speeds of the two photosensitive drums, and it affects an image formed. That component is basic one created by each one full rotation of each photosensitive drum. Therefore, as shown in FIG. 15, the image becomes wide or narrow at the period of each one full rotation of the photosensitive drum. In other words, the speed fluctuation occurring due singly to the photosensitive drum affects the images formed on the plurality of photosensitive drums.
What has been described above will be further described by the use of expressions as follows.
In FIG. 17, V.sub.3 and V.sub.4 relate at the period of each one full rotation of the photosensitive drum and are driven by the same driving belt 42 and thus, the speed of each photosensitive drum at the same time is V.sub.3 =V.sub.4 =sin .theta. (.theta. being the rotation angle of the photosensitive drum). However, an image formed at the speed V.sub.4 after the paper has advanced by a distance L between the photosensitive drums overlaps an image formed at the speed V.sub.3 on the sheet, and therefore the amount of deviation between the images on the two photosensitive drums can be expressed as follows. In the expression below, L is the distance between the photosensitive drums, V is the speed of the sheet, t=L/V is a constant, and K=2.multidot.sin (t/2) is a constant.
The amount of deviation between the images on the two photosensitive drums=(image formed at V.sub.3)-(image formed at V.sub.4) ##EQU1##
As shown by the above expression, the amount of deviation is a trigonometric function which can be represented by the rotation angle of the photosensitive drum, and coincides with the behavior shown in FIG. 15.
That is, in the above-described example of the prior art, all of a plurality of photosensitive drums in which the distance from the center of rotation to the surface changes due to the accuracy or the like of parts during each one full rotation of the drums are driven by a single drive source disposed at a location spaced apart from the group of photosensitive drums which are driven members, through a mechanical element. Therefore, it is difficult for the speed controlling function of the drive source itself to effectively affect each photosensitive drum. Therefore, the images formed by the use of the plurality of photosensitive drums were ones fluctuated during each rotational period of the photosensitive drums. Particularly, when full color images were formed, their behavior appeared as the color misregistration of the images and caused a reduction in the quality of image.